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From "Rodenburg, Jeff" <jeff.rodenb...@teamaol.com>
Subject Re: Consumer group concept
Date Tue, 12 Jun 2012 22:19:59 GMT
Thanks Joel, this makes sense now. I've been walking through the design document most of the
day and was trying to reconcile the difference between consumer streams and partitions (not
sure why I tied the two together previously.)

Pretty sure I'm following this now, thank you for the comments.

-j



On Jun 12, 2012, at 2:24 PM, Joel Koshy wrote:

> Hi Jeff,
> 
> Load balancing is done by range partitioning the available partitions for
> the topic across the consumer processes (streams). The algorithm is given
> at the very end of the design document:
> http://incubator.apache.org/kafka/design.html - but here's a quick example.
> If you have four nodes, and two message streams per node (i.e., each node's
> consumer config is "foo":2) this means there are eight consumer streams in
> total. The available partitions for "foo" are allocated to these eight
> streams using the rebalancing algorithm. For e.g,. if there are eight
> available partitions on the brokers then each consumer stream with get one
> partition. If there are fewer than eight, some of the consumer streams will
> not get any data. If there are more than eight, then some streams will get
> more than one partition (if # partitions % # streams == 0 then it will be
> evenly spread, and skewed otherwise).
> 
> Thanks,
> 
> Joel
> 
> On Tue, Jun 12, 2012 at 1:55 PM, Rodenburg, Jeff <jeff.rodenburg@teamaol.com
>> wrote:
> 
>> Great, I'm running the quick start and can see that in operation.
>> 
>> Ok, last question on this thread:
>> 
>>> So if you have two consumer groups consuming a topic, and each consumer
>> group has 4 machines in it, then a message published to this topic would be
>> delivered to one machine in each of the two groups.
>> 
>> How is topic load-balancing for consumers handled?  For example, if a
>> consumer group has 4 machines in it (consumer per machine), in reality only
>> one machine in the group is actually working.  If I want multiple machines
>> handling items in a topic, how is that approach handled? I could see
>> producers generating more topics, and consumers subscribing to those
>> (making a high-volume topic more granular).  What's best practice when
>> consumer tasks on topic messages need to be handled by multiple consumers?
>> 
>> -Jeff
>> 
>> 
>> 
>> 
>> 
>> On Jun 12, 2012, at 11:46 AM, Jay Kreps wrote:
>> 
>>> Basically the rule is this "every message sent to the topic is delivered
>> to
>>> one machine/process in each consumer group". So if you have two consumer
>>> groups consuming a topic, and each consumer group has 4 machines in it,
>>> then a message published to this topic would be delivered to one machine
>> in
>>> each of the two groups.
>>> 
>>> -Jay
>>> 
>>> On Tue, Jun 12, 2012 at 11:34 AM, Rodenburg, Jeff <
>>> jeff.rodenburg@teamaol.com> wrote:
>>> 
>>>> Thanks for the info, Jun.
>>>> 
>>>>> if you just want each message to be consumed by a consumer, not a
>>>> particular one
>>>> 
>>>> What is intended to be a particular consumer? Something on the order of
>>>> Consumer #3 within a group needs message #123?
>>>> 
>>>> Ok, next question:
>>>> 
>>>> What is the relationship between topics and consumer groups? More to the
>>>> point, can I have multiple consumer groups that all consume the same
>> topic?
>>>> For example, assume a set of producers are publishing to the topic
>> "ABC".
>>>> Suppose I have multiple processes that take action on a given ABC
>> message
>>>> -- process 1 handles billing, process 2 handles file management,
>> process 3
>>>> handles history/archiving, etc.  Can I structure multiple groups that
>>>> consume the same topic? How does partitioning work at that point?
>>>> 
>>>> 
>>>> 
>>>> 
>>>> On Jun 12, 2012, at 10:11 AM, Jun Rao wrote:
>>>> 
>>>>> Jeff,
>>>>> 
>>>>> Your understanding is correct. Operational wise, we have some jmx that
>>>>> gives consumer stats per topic. There is also a tool CheckOffsetLag
>> that
>>>>> tells you how far behind a consumer is. For coordination btw producers
>>>> and
>>>>> consumers, if you just want each message to be consumed by a consumer,
>>>> not
>>>>> a particular one, there is no coordination needed.
>>>>> 
>>>>> Thanks,
>>>>> 
>>>>> Jun
>>>>> 
>>>>> On Tue, Jun 12, 2012 at 9:58 AM, Rodenburg, Jeff <
>>>> jeff.rodenburg@teamaol.com
>>>>>> wrote:
>>>>> 
>>>>>> Hi all -
>>>>>> 
>>>>>> Just getting familiar with Kafka, and learning about consumer groups.
>>>>>> Hoping someone can provide some context here.
>>>>>> 
>>>>>> As I understand it, consumers register with the broker and consume
a
>>>>>> topic. Multiple consumers can consume a single topic, as a consumer
>>>> group.
>>>>>> Each consumer actually gets a partition of messages, so there is
no
>>>> overlap
>>>>>> -- a single consumer within a group will receive a message on its
>>>>>> topic/partition.  Consumer rebalancing is the process whereby members
>>>> of a
>>>>>> consumer group are added and/or dropped from the group, and partitions
>>>> are
>>>>>> sorted/reassigned to the current consumer group members.
>>>>>> 
>>>>>> Some questions:
>>>>>> 
>>>>>> *   Is this accurate? What am I missing?
>>>>>> *   Operationally, is consumer "failover" basically service monitoring
>>>> at
>>>>>> the consumer process level?
>>>>>> *   How much coordination is required between producers and consumers
>>>>>> around partitioning? (Automated, configuration, etc.)
>>>>>> *   How are topics monitored for SLA on throughput/load, i.e. spinning
>>>> up
>>>>>> consumers as needed for topic message spikes?
>>>>>> 
>>>>>> Appreciate any further information and/or context anyone can share.
>>>>>> 
>>>>>> cheers,
>>>>>> Jeff
>>>>>> 
>>>> 
>>>> 
>> 
>> 


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